Method for providing protective surfaces



United States Patent 3,118,792 METHUD FQR iRGVlDlNG K RQTECTIVE SURFACES Bertha S. Tattle, Concord, and Walter A. Vittands, Jamaica Plain, Mesa, assignors to .l. N. Tattle, End,

Cambridge, Mass, a COREEOIfifiOH of Massachusetts No Drawing. Filed Nov. 4, 1960, Ser. No. 66,919

20 Claims. (Cl. l43-6.l4)

This, invention relates to a novel and improved method for treating phosphate-coated surfaces of iron and steel to provide superior corrosion resistance. This application is a continuation-in-part of our copending application Serial No. 833,985, filed August 17, 1959, now abancloned.

it has long been known that zinc, cadmium and ferrous surfaces may be rendered more corrosion resistant by a treatment with phosphoric acid and/ or phosphates of the metals of manganese to cadmium in the electromotive series. Since the essential ingredient embodied in such treatments is the phosphate radical, the processes are generally referred to as phosphate treatments. Such phosphate treatments have, however, not provided as high a degree of corrosion resistance as might be desirable, without further treatments, such as oiling, waxing or painting. Some of such further treatments generally result in objectionable dimensional changes and therefore, in many instances, do not provide a practical approach for improving the corrosion resistance of phosphate coated articles. A practical method of treating a phosphate coated surface in order to improve its corrosion resistance and which Will not produce appreciable dimensional changes is described and claimed in United States Patent No. 2,478,954, issued August 16, 1949. The process of the aforementioned patent involves the treating of a phosphate coated surface with an aqueous solution of a stannous salt which is water soluble and Water stable, and particularly involves the treatment of a phosphate coated surface with an aqueous solution of stannous chloride. Such a treatment of a phosphate coated surface will hereinafter generally be referred to as a stannous salt treatment.

The stannous salt treatment of a phosphate coated surface provides a coating believed to comprise an insoluble tin-metal-phosphate complex which has a corrosion resistance markedly exceeding that of a simple phosphate coating. if desired, a dye compatible with the stannous salt solution may be added thereto in order to impart color to the coating. The color so imparted is durable and cannot be rubbed or washed off. Such dyeing also imparts greater corrosion resistance to the coatings.

One operational criteria of a stannous salt treatment is that the stannous salt solution should not have an acidity which is sufficient to strip the phosphate coating. Another criteria is that the acidity of the stannous salt treating bath should be adjusted so that the phosphate undercoat will be attacked and the conversion to a complex stannous phosphate will be accomplished substantially at the same time. Thus, the control of the acidity of the stannous salt solution is an important factor in the process.

In practice, it has been found that the acidity of stannous salts and particularly, stannous chloride, obtained from different sources may vary widely. Also, many such salts have the disadvantageous characteristic of deteriorating in storage. Further, when a stannous salt bath is brought to the boiling point and held there for a prolonged period, the stannous salt reacts with the water of the solution as well as with the oxygen of air. By both reactions a precipitate is formed, and hydrochloric acid is set free. This acidity can build up and reach a point where it may strip the phosphated coating and cause discoloration of the article being treated. However, some "ice acidity is necessary for the desired reaction to take place, and the degree of acidity needs to be controlled to an extent determined by the character of the phosphate base being treated. For example, a relatively neutral stannous salt bath forms a uniform grey coating on a standard zinc base phosphate surface in a period of from two of five minutes. However, other types of coating such as manganese phosphate coatings require greater acidity to accomplish the conversion in the same time. Further a more acid bath will accomplish the conversion in a shorter time. It has been observed that the faster the reaction, or in other words, the more acid the bath, the more exacting must be the control of the acidity level of the bath.

The problem of providing a stannous salt solution of the proper initial acidity for treatment of zinc phosphate coated parts was, to a large extent, solved by the development of a process for producing a relatively pure aqueous stannous chloride concentrate, which is substantially free from acid and which is stable over long periods of time. This process is described in United States Patent No. 2,868,623, issued January 13, 1959. The process of this last mentioned patent provides a neutral stannous salt concentrate which is readily diluted to provide a treating bath which requires little, if any, adjustment of the acidity thereof prior to use thereof in connection with parts provided with a thick Zinc phosphate coating. While the use of neutral stannous salts, as described in the last mentioned patent, substantially reduced the'problem of initial adjustment of the stannous salt treating bath, there still remained the problem of maintaining the bath at the proper level of acidity during use.

The skill required to control the acidity of the stannous salt bath during use thereof, may be reduced by improvement of the neutral stannous salt concentrate described in the last mentioned patent, by means of the addition of water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups, or the lactone thereof. This improvement particularly in connection with tartaric acid is described in US. Patent No. 2,854,367, issued September 30, 1958. The stannous salt neutral concentrate containing a water soluble aliphatic polyhydroxy acid of the type described may be stored substantially indefinitely without deterioration. When such an improved concentrate is used to provide a treating bath, a lesser amount of control is required to maintain the desired acidity of the bath during use, and the process of this last mentioned patent also made a two step dyeing process more feasible. However, even with the use of the improved stannous salt concentrate there is a necessity for the operator to add soda ash, or the like to the bath in order to maintain the proper acidity. It is desirable to further reduce the necessity for frequent additions of soda ash to the bath thus further reducing the skill necessary to operate the bath, as well as providing other advantages.

A stannous salt treating bath prepared in accordance with the teachings of Patent No. 2,854,367 provides satisfactory results with respect to a surface having a relatively thick zinc phosphate coating, as regards corrosion resistance thereof. Further, the same bath, when used to treat a manganese phosphate coated surface will take a much longer time and will increase the salt spray resistance of the manganese phosphate coated surface from say one hour to approximately ten hours. While this in an improvement in the salt spray resistance of the manganese phosphate coated article, the resulting salt spray resistance is not as great as the resistance obtainable with a stannous salt treated zinc phosphate coated surface. Because of the added expense of the manganese phosphate coating, this treatment has, in the past, been used primarily where it was desired to impart a wear-resistant characteristic to the surface and not primarily for improved corrosion resistance.

It is the object of the present invention to provide an improved stannous salt treatment of phosphate coated articles which will materially reduce the necessity for adjustment of the acidity of a stannous salt bath, especially after prolonged periods of idling and will reduce the skill necessary to control the acidity of the stannous salt solution, particularly in the case of more acid solu tions. Included within this object is the provision of an improvement of the rustproofing obtained with a stannous salt treatment of zinc phosphate coated parts and also the provision of a practical and effective production method for the stannous salt treatment of manganese phosphate coated articles, which will markedly improve the corrosion resistance of the manganese phosphate coating While at the same time will not eliminate the wearresistance characteristics thereof.

Other objects will be in part obvious, and in part pointed out more in detail hereinafter.

In carrying out the process of this invention, a suitable metal article is first treated to provide a phosphate coating thereon. The phosphate coated article is then treated with an aqueous treating solution comprising a stannous salt. Although as disclosed in the aforementioned Patent No. 2,478,954, various water soluble stannous salts may be used, we prefer to use a stannous chloride prepared as a relatively pure substantially neutral concentrate in the manner taught in Patent No. 2,868,623. Such a concentrate contains 1 kilogram or more of stannous chloride (SnCI -ZH O) per liter of solution and may also contain an appropriate amount of requisite ingredients. We prefer to prepare the stannous salt treating bath by diluting the stannous salt concentrate with water in an amount which may vary from cc. to 1000 cc. of the concentrate per liter of treating solution with a preferred treating bath comprising 30 to 50 grams of stannous chloride per liter. Thus, the treating solution comprises an aqueous solution of stannous salt, in which the stannous salt is present in an amount of from approximately 10 grams to 1000 grams per liter of treating solution. It is further preferred that the stannous salt bath be of the type taught by Patent No. 2,854,367, wherein the bath further comprises a water soluble aliphatic polyhydroxy acid of the type described, or the lactone thereof, the acid or lactone being present in an amount of from 0.1% to by weight of the stannous salt. We prefer to use a treating solution comprising tartarinc acid in the amount of approximately 0.5 gram per liter. Further, we prefer to use a treating solution to which has initially been added a lead salt, and preferably lead chloride, in an amount of from .2% to 100% by weight of the stannous salt. In the preferred bath comprising -50 grams per liter of stannous chloride and approximately 0.5 gram per liter of tartaric acid, a preferred range of the lead salt is from .2 to 20 grams per liter. The lead salt is preferably added to the treating bath rather than to the stannous salt concentrate inasmuch as the lead salt is not sufficiently soluble in the concentrate.

In accordance with one aspect of the invention, we add metallic lead to the stannous salt treating bath with the lead being preferably in the form of sheets, bars or the like, suspended in the bath, which is preferably containcd in a stainless steel tank. It will be apparent that while lead is specified, the use of a lead alloy is also within the meaning of this term. It is preferred that the surface area of the lead exposed to the treating solution be at least approximately one square inch per liter of solution. It has been found that the contacting of metallic lead with the solution in the manner described will tend to maintain the acidity of the bath at the desired level, although under production conditions where the bath may be idled over long periods, some neutralizing with soda ash or the like may be required. However, the

4 amount of adjustment is relatively small as compared with prior processes and becomes less as the bath ages. The reasons for the maintenance of the acid level of the bath by the addition of metallic lead is not completely understood but is believed due to the attacking of the lead by the free acid formed in the bath, which acid is primarily hydrochloric acid, with the result that the free acid is neutralized and lead chloride is formed. The lead chloride formed is believed to buffer the bath and inhibit the buildup of excessive acidity thus eliminating stripping or staining of the phosphate coated surface. The initial addition of the lead chloride is preferred as it is believed that it provides an initial buffering of a new bath. The initially added lead chloride, which is believed to be used up during the process, is believed to be replenished by the lead chloride formed by the action between the stannous salt solution and the metallic lead. As will be apparent from subsequently described examples of this process, the acidity control of the stannous salt bath by the method of this invention prevents the accumulation of detrimental amounts of NaCl in the bath inasmuch as the introduction of excessive amounts of soda ash is eliminated.

While the improved process of this invention provides advantages in connection with zinc phosphate coated articles, by simplifying the control of the acidity of the stannous salt solution, it has a further distinct advantage in that it provides a very marked improvement in the corrosion resistance of manganese phosphate coated articles. it has been found that by raising the acidity of the stannous salt treating bath of the type disclosed in Patent No. 2,854,367 the bath becomes sufliciently vigorous to react with a manganese phosphate coating and provide a remarkable improvement in the salt spray resistance of the surface treated. However, at the higher acidity level the oxidation of a stannous chloride solution which yields hydrochloric acid, is catalytically accelerated by hydrogen ions. While adjustment of the acidity level of the bath by the addition of soda ash might theoretically be possible, from a practical standpoint, the additions of soda ash would be all too frequent for a production process. Also, this frequent addition of soda ash would require a very high degree of skill on the part of the operator in order to control the acidity. Also, the attendant contamination of the stannous chloride solution by sodium ions is undesirable.

The addition of metallic lead to a stannous salt bath having the higher acidity necessary to convert a manganese phosphate coated article, results in a reduction of the skill required to control the acidity of the bath as well as a reduction in the frequency of adjustments of the bath during use to a point where the stannous salt treatment of a manganese phosphate coated article on a production basis is practical. We have found that the contacting of metallic lead with a stannous salt bath, which has initially been adjusted to the proper acidity, substantially reduces subsequent adjustment of the acidity of the bath even after long idling periods of the bath. For example, in a prior process for the stannous salt solution of manganese phosphate coated articles utilizing a bath of the type disclosed in Patent No. 2,854,367 it was customary to adjust the acidity of the bath at the beginning of the working day and also several times during the day. The acidity of the bath was particularly diflicult to control and the resulting production left much to be desired. The frequent addition of soda ash resulted in undesirable build up of sodium chloride and the rustproofing obtained was insufficient to warrant the expense of the process. With a stannous salt bath of this invention, it has been found necessary to adjust the acidity of the bath at the beginning of a working day. The particular bath concerned was operated intermittently and at times stood idle for a week or more. It is believed possible that with continuous operation of the bath, the addition of soda ash may even be further reduced or even substantially eliminated. Further,

the rustproofing obtained from the stannous salt bath of this invention is remarkably greater. The advantage of this aspect of the invention may best be illustrated by an exemplary comparison of the salt spray resistance of manganese phosphate coated articles treated with a stannous salt solution, as described in Patent No. 2,854,367, and a manganese phosphate coated article treated in accordance with the improved stannous salt treatment of the present invention.

Generally speaking, in the case of manganese phosphate coated articles treated with a solution of a type covered by Patent No. 2,854,367, improvement in salt spray resistance of from one hour to 6-12 hours may be obtained. However, in the case of manganese phosphate coated articles treated by the improved process of this invention, salt spray resistances of over 200 hours have been obtained.

Specific examples of preferred procedures for practicing the process of this invention with respect to zinc and manganese phosphate coatings and specific examples of the improved results obtained are as follows:

EXAMPLE A Three stannous salt treating baths were prepared by diluting a substantially neutral concentrated stannous chloride solution to yield a bath containing 36 grams of stannous chloride per liter. In two of these baths were inserted lead sheets and to one of these two baths grams per liter of lead chloride were added. The baths were operated identically in connection with the processing of Zinc base phosphated parts, and the amounts of 2 normal soda ash solution required for the adjustment during two weeks were recorded. The bath Without any additions of lead or lead chloride required 42 ml. per liter, the bath with a lead sheet required 33.5 ml. per liter, and the bath with a lead sheet and lead chloride required ml. per liter.

EYAMPLE B The procedure set forth in Example A was followed except that 0.5 g. per liter of tartaric acid were added to each bath. These baths were operated for one week all the time being kept well adjusted. The amounts of 2 N soda ash required for each bath were recorded. The bath without any lead or lead chloride required 18.5 ml. per liter, the bath with a lead sheet required only 13 ml. per liter, and the bath with a lead sheet and lead chloride required 10.5 ml. per liter.

EXAMPLE C The procedure set forth in Example B was followed except that 0.7 ml. concentrated hydrochloric acid per liter of stannous solution were added to each bath. The baths were operated for one week processing manganese base phosphated parts and recording the amounts of 2 N soda ash solution required to keep the baths well adjusted. The bath without any lead required 17 ml. per liter, the bath with a lead sheet required only 5 ml. per liter, and the bath with a lead sheet and lead chloride required 3 ml. 2 N soda ash per liter.

EXAMPLE D The procedure set forth in Example C was followed and the manganese base phosphated parts treated in all baths were tested in the saltspray chamber. Those parts treated in the bath without lead gave inconsistent results and the first sign of rust appeared on the average after 10-12 hours, while parts treated in the baths containing lead or lead and lead chloride were all perfect after 100 hours.

The use of lead salts has been described above in connection only in combination with metallic lead wherein the initially added lead salt provides a buffering action during the initial life of the bath. It has been found that the addition of chloride salts, of lead as well as certain other metals to a stannous salt treating bath of the type disclosed in Patent No. 2,854,367 will provide improved rustproofing of Zinc base phosphate coated articles. The reason for this is not completely understood. However, it is believed in part due to a presence of lead in the final coating, inasmuch as the lead has been observed in the final coating of the article. More specifically, this aspect of the invention contemplates a treatment of phosphate coated articles with a treating solution comprising a stannous salt and a salt of one of the group of water soluble salts of lead, cadmium, manganese, and zinc, to which solution has been added a water soluble aliphatic polyhydroxy acid or the lactone thereof. We prefer to use a treating solution comprising tartaric acid in the amount of approximately 0.:5 gram per liter.

We have found that a stannous salt treatment, when carried out with a treating solution comprising both a proportion of a salt of the group set forth above and a proportion of an aliphatic polyhydroxy acid or the lactone thereof, provides a corrosion-resistant coating which is more effective than that provided by a similar treatment in which the treating solution does not comprise the aliphatic polyhydroxy acid or the lactone thereof or the metallic salts.

Procedures illustrative of the foregoing are as follows:

Example I To an aqueous treating solution prepared as described in Example I of the US. Patent No. 2,854,367 and containing per liter 38 grams stannous chloride and 0.5 gram tartaric acid was added 15 grams per liter of lead chloride. Zinc phosphate coated ferrous metal parts were treated in this solution for three to five minutes at boiling temperature. The parts were then salt sprayed with a 20% salt solution at F. for forty-seven hours. Similar parts which were treated in the above mentioned treating solution prior to the addition of the lead chloride had a salt spray rating of six (definite rusting on the surface) while the parts treated after the addition of lead chloride to the solution showed a rating of eight (very slight rusting). In the salt spray rating scale the rating one is used to define a totally rusted surface and the rating ten is used to denote a perfect, unrusted surface.

Example II The procedure set forth in Example 1 was followed except that 20 grams per liter of Zinc chloride were added instead of the lead chloride. Zinc phosphate coated parts treated in the bath prior to the addition of the zinc chloride had a salt spray rating of seven after 71 hours of testing, while the phosphate coated parts treated in the bath following the addition of the zinc chloride had a salt spray rating of eight after the same time of testing.

xample III The same procedure set forth in Example I was followed except that 0.5 gram per liter of cadmium chloride were added instead of the lead chloride. Zinc phosphate coated parts treated in the bath prior to the addition of the cadmium chloride had a salt spray rating of eight after fortyight hours of testing, while similar parts treated in the bath after the addition of the cadmium chloride had a salt spray rating of ten after the same period of testing.

Example IV The same procedure as set forth in Example I was followed except that 40 grams of manganese chloride tetrahydrate were added instead of the lead chloride. Zinc phosphate coated parts treated in the bath prior to the addition of the manganese chloride had a salt spray rating of eight after forty-eight hours of testing, while similar parts treated in the bath after the addition of manganese chloride had a salt spray rating of nine after the same time of testing.

Example V The procedure as set forth in Example I was followed except that 20 grams per liter of a mixture of manganese chloride and lead chloride in a proportion of 1:1 were added instead of lead chloride. Zinc phosphate coated parts treated in the bath prior to the addition of manganese chloride lead chloride mixture had a salt spray rating of six after forty hours of testing, while similar parts treated in the bath after the addition of the above mentioned mixture had a rating of nine after the same period of testing.

Phosphate coated articles treated in accordance with the present invention may be dyed subsequent to the stannous salt treatment, by immersion of the articles in a hot solution of certain dyes followed by rinsing in water. All of the tests referred to above were made made on unoiled test pieces so that the efiicacy of the various treatments could be determined more accurately. It is to be understood, however, that such supplementary treatments as oiling, waxing, etc. may be used, if desired, to substantially increase the corrosion resistance of the parts treated in accordance with the present invention.

With respect to the use of metallic lead in the control of the acidity of the stannous salt bath, it has been found that the stabilizing action of the lead sheets on the treating solution diminishes if metallic tin becomes deposited on the lead by a side reaction between the stannous chloride solution and the lead; however, by brushing with a steel wire periodically Whenever necessary, the activity of the lead sheets can be restored easily to their effectiveness.

We claim:

1. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous treating solution of a stannous salt wherein the solution comprises at least 10 grams per liter of stannous salt, the solution having metallic lead in the form of a sheet at least partially immersed therein.

2. An article having a protective coating comprising an insoluble tin-lead-metal-phosphate complex said coating produced in accordance with the method of claim 1.

3. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous treating solution of stannous chloride wherein the solution comprises at least 10 grams per liter of stannous chloride, the solution having metallic lead in the form of a sheet at least partially immersed therein.

4. In the method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface wherein the phosphate coated article is subjected to contact with an aqueous treating solution of a stannous salt wherein the solution comprises at least 10 grams per liter of stannous salt, the improvement of initially adding to the treating solution from 0.2% to 100% by weight of the stannous salt of a water soluble lead chloride, and of maintaining said solution in contact with metallic lead in the form of sheets at least partially immersed in the solution, thereby to tend to maintain the acidity of the treating solution at a predetermined level.

5. In the method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface wherein the phosphate coated article is subjected to contact with an aqueous treating solution of stannous chloride wherein the solution comprises at least 10 grams per liter of strannous chloride, the improvement of initially adding to the treating solution from 0.2% to 100% by weight of the stannous salt of a water soluble lead chloride, and of maintaining said solution in contact with metallic lead in the form of sheets at least partially immersed in the solution, thereby to tend to maintain the acidity of the treating solution at a predetermined level.

6. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjcctin g said phosphate coated article to contact with an aqueous treating solution comprising from 30 to 50 grams per liter of stannous chloride, approximately 0.5 gram per liter of tartaric acid, and from 2 to 20 grams per liter of lead chloride while maintaining the solution in contact with metallic lead in the form of sheet at least partially immersed in the solution, the solution having an acidity insufficient to strip said phosphate coating.

7. The method of treating a metal article having an insoluble manganese phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting the manganese phosphate coated article to contact with an acidic aqueous stannous salt treating solution which is in contact with metallic lead and wherein the solution comprises at least 10 grams per liter of stannous salt.

8. An article having a protective consisting essentially of an insoluble tin-lead manganese-phosphate complex said coating produced in accordance with the method of claim 7.

9. The method of treating a metal article having an insoluble manganese phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting the manganese phosphate coated article to contact with an acidic aqueous stannous chloride treating solution which is in contact with metallic lead and wherein the solution comprises at least 10 grams per liter of stannous salt.

10. The method of treating a metal article having an insoluble manganese phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting the manganese phosphate coated article to contact with an acidic aqueous stannous salt treating solution comprising at least 10 grams per liter of stannous chloride to which has been initially added from 0.2% to by weight of the stannous salt of water soluble lead chloride, and maintaining said solution in contact with metallic lead, thereby to tend to maintain the acidity of the treating solution at a predetermined level.

11. The method of treating a metal article having an insoluble manganese phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting the manganese phosphate coated article to contact with an aqueous stannous salt treating solution comprising at least 10 grams per liter of stannous salt, which solution has been rendered sufficiently acidic that it will simultaneously attack and convert the manganese phosphate coating, initially adding from 0.2% to 100% by weight of the stannous salt of a water soluble lead chloride, and maintaining said solution in contact with metallic lead, thereby to tend to maintain the acidity of the treating solution at a predetermined level.

l2. The method of treating a metal article having an insoluble manganese phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting the manganese phosphate coated article to contact with an acidic aqueous stannous salt treating solution comprising at least 10 grams per liter of stannous salt and which further comprises from 0.1% to 50% by weight of the stannous salt of a water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups and from 0.2% to 100% by weight of the stannous salt of water soluble lead chloride, and maintaining said solution in contact with metallic lead, thereby to tend to maintain the acidity of the treating solution at a predetermined level.

13. The method of treating a metal article having an insoluble manganese phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting the manganese phosphate coated article to contact with an aqueous stannous chloride treating solution which has been rendered sufliciently acidic that it will simultaneously attack and convert the manganese phosphate coating, said treating solution consisting essentially of 30 to 50 grams per liter of stannous chloride, approximately 0.5 gram per liter of a Water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups and from 2 to 20 grams per liter of lead chloride, and contacting the treating solution with metallic lead.

14. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous solution of a stannous salt wherein the solution comprises at least grams per liter of stannous salt, from 0.1% to 50% by weight of the stannous salt of a water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups, and from 0.2% to 100% by weight of the stannous salt of water soluble lead chloride, the solution having an acidity insuflicient to strip said phosphate coating.

15. An article having a protective coating comprising an insoluble tin-lead-metal-phosphate complex, said coating produced in accordance with the method of claim 14.

16. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous solution of a stannous salt wherein the solution comprises at least 10 grams per liter of stannous salt, from 0.1% to 50% by weight of the stannous salt of a water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups, and from 0.2% to 100% by weight of the stannous salt of Zinc chloride, the solution having an acidity insufficient to strip said phosphate coating, thereby to form an insoluble tin-metal-phosphate complex having a corrosion resistance exceeding the original phosphate coating.

17. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous solution of a stannous chloride wherein the solution comprises at least 10 grams per liter of stannous chloride, from 0.1% to 50% by weight of the stannous chloride of a water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups, and from 0.2% to 100% by weight of the stannous chloride of a lead chloride, the solution having an acidity insufficient to strip said phosphate coating, thereby to form an insoluble, tin-metalphosphate complex having a corrosion resistance exceeding the original phosphate coating.

18. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous treating solution comprising from 30 to grams per liter of stannous chloride, approximately 0.5 gram per liter of tartaric acid, and from 2 to 20 grams per liter of lead chloride, the solution having an acidity insufiicient to strip said phosphate coating.

19. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous solution of a stannous salt wherein the solution comprises at least 10 grams per liter of stannous salt, from 0.1% to 50% by Weight of the stannous salt of a water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups, and from 0.2% to by weight of the stannous salt of water soluble cadmium chloride, the solution having an acidity insuflicient to strip said phosphate coating.

20. The method of treating a metal article having an insoluble metal phosphate coating thereon produced by chemical reaction of the phosphate radical with the metal surface, which comprises subjecting said phosphate coated article to contact with an aqueous solution of a stannous salt wherein the solution comprises at least 10 grams per liter of stannous salt, from 0.1% to 50% by weight of the stannous salt of a water soluble aliphatic polyhydroxy acid having at least one carboxyl group and at least two hydroxyl groups, and from 0.2% to 100% by weight of the stannous salt of water soluble manganese chloride, the solution having an acidity insuflicient to strip said phosphate coating.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE METHOD OF TREATING A METAL ARTICLE HAVING AN INSOLUBLE METAL PHOSPHATE COATING THEREON PRODUCED BY CHEMICAL REACTION OF THE PHOSPHATE RADICAL WITH THE METAL SURFACE, WHICH COMPRISES SUBJECTING SAID PHOSPHATE COATED ARTICLE TO CONTACT WITH AN AQUEOUS TREATING SOLUTION OF A STANNOUS SALT WHEREIN THE SOLUTION COMPRISES AT LEAST 10 GRAMS PER LITER OF STANNOUS SALT, THE SOLUTION HAVING METALLIC LEAD IN THE FORM OF A SHEET AT LEAST PARTIALLY IMMERSED THEREIN. 